Noise attenuation panel

ABSTRACT

A noise attenuation panel having a backing sheet, a facing sheet and a cellular core having a multiplicity of open-ended juxtaposed cells. The backing sheet extends across the ends of the cells at the rear of the core and the facing sheet extends across the ends of the cells at the front of the core. The facing sheet is made of a porous permeable thermoplastics material. The porous permeable thermoplastics material for the facing sheet is produced by powder sintering a thermoplastics material.

This application is a continuation of PCT/GB89/00841, filed Jul. 24,1989, designating the United States.

This application is a continuation of PCT/GB89/00841, filed Jul. 24,1989, designating the United States.

The present invention relates to noise attenuation panels and isparticularly, although not exclusively, concerned with noise attenuationpanels for use in the attenuation of noise in aero engines.

It has previously been proposed to provide noise attenuation panels forreducing turbine engine noise, the panels being formed as integral partsof the aero engine structure. For example, the panels may form part of acowling surrounding a turbine engine inlet duct or be placed adjacent ahigh turbulence region of the fan of a turbo fan engine.

In particular, prior specification GB 2056367A describes a process forproducing broad band noise attenuation panels for use in suchenvironments, where they are used as skin surfaces adjacent the engine.

The panel of GB 2056367A comprises a central honeycomb core having aplurality of open-ended juxtaposed cells, an imperforate backing sheetextending across the ends of the cells at the rear thereof, a perforateinner facing sheet extending across the ends of the cells at the frontof the core and an outer facing sheet of fibrous or woven materialbonded to the inner facing sheet, which is said to improve the soundattenuation characteristics.

The preferred material for the core and the backing and inner facingsheets of the panel of GB 2056367A is aluminium due to its weight versusstrength and cost characteristics, although it is envisaged that othermetals or materials could be used where these requirements are notconsidered.

The inner facing sheet for the panel of GB 2056367A is perforated with aplurality of small perforations and the outer facing sheet is adhesivelybonded to the surface of the perforated inner facing sheet. Thepreferred material for the woven outer sheet is stainless steel, whichis woven as a Dutch twill fabric, although it is stated that other typesof material and weave pattern may be utilised.

It has furthermore previously been proposed in patent specification GB2122540A to provide a noise attenuation panel of the form described inGB 2056367A, except insofar that the apertured inner facing sheet isformed from a carbon fibre/resin matrix composite material. Inparticular, the apertured inner facing sheet is formed as an open weavematerial in which the apertures are constituted by hexagonal openingsbetween three sets of carbon threads mutually arranged at 60° to oneanother. The porous woven outer facing sheet is, however, made ofstainless steel.

The use of a composite material for the apertured inner facing sheet ofthe panel disclosed in GB 2122540A has, it is stated, advantages overthe use of materials such as aluminium. The apertures can for example beformed while the resin matrix of the composite is in a partially curedcondition, leading to a far less expensive process than formingperforations in a metal sheet. Furthermore, the resin matrix of thecomposite material could, it is stated, itself provide an adhesivemedium by which the porous outer facing sheet is secured to theapertured inner facing sheet.

While the noise attenuation panel disclosed in GB 2122540A has beenfound to be successful for use in aero engine environments, it suffersfrom a number of disadvantages. In particular, it has been found thatthe use of a woven material for the porous outer facing sheet gives riseto a relatively high resistance to high speed air flow over the surfaceand that sound attenuation is effective over a relatively narrow band offrequencies. Furthermore, the problem of metal galvanic corrosion isresent where the material used is stainless steel.

It is an object of the present invention to provide a noise attenuationpanel which does not suffer from the above-mentioned disadvantages ofthe noise attenuation panels previously proposed and as hereinbefore setforth or does not suffer to the same extent from those disadvantages.

According to a first aspect of the present invention there is provided anoise attenuation panel comprising a backing component part, a facingcomponent part and a cellular component part having a multiplicity ofopen-ended juxtaposed cells, the backing component part extending acrossthe ends of the cells of the cellular component part at the rear thereofand the facing component part extending across the ends of the cells ofthe cellular component part at the front thereof, characterised in thatthe facing component part comprises or includes an outer facing sheetwhich is made of a porous permeable thermoplastics material.

Preferably, the porous permeable thermoplastics material is produced bypowder sintering of a thermoplastics material.

Examples of suitable thermoplastics materials include polyether etherketone, polyaromatic ketone, polyphenylene sulphide, polyamide-imide,thermoplastic polyimide, polyether-imide, polyurethane and polyethylene.

In all embodiments of the invention hereinafter to be described, thebacking component part is imperforate and made of an impermeable sheetmaterial which may be made from a non-porous thermoplastics material. Insome embodiments of the invention hereinafter to be described, thecellular component part is also made of an impermeable sheet materialwhich may also be a non-porous thermoplastics material.

In a first embodiment of the invention hereinafter to he described, thefacing component part further includes an inner support sheet which isperforated and which provides structural support from the outer facingsheet.

The perforated inner support sheet may take any one of a number ofdifferent forms and may be made from any one of a number of differentmaterials. It may, for example, be formed from (i) a carbon/epoxy resincomposite material as proposed in GB 2122540A, (ii) a perforatedaluminium alloy sheet, or (iii) an open weave material produced from anepoxy resin impregnated carbon fibre. It may also be made from athermoplastics material.

In a second embodiment of the invention hereinafter to be described, thecellular component part comprises a front cellular element having amultiplicity of open-ended juxtaposed cells, a rear cellular elementhaving a further multiplicity of open-ended juxtaposed cells and aseptum element which extends across the ends the cells of the rearcellular element at the front thereof and the ends of the cells of thefront cellular element at the rear thereof and which is made of a porouspermeable thermoplastics sheet material.

The porous permeable thermoplastic sheet material forming the septumelement may be produced by powder sintering of a thermoplasticsmaterial.

The walls of the cells of the rear cellular element are preferably madeof a non-porous impermeable sheet material whereas the cells of thefront cellular element may in accordance with a further embodiment ofthe invention be made of a porous permeable thermoplastics material.

According to a second aspect of the present invention, there is providedan aero engine including as a structural part of the engine a noiseattenuation panel according to the first aspect of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described with reference to theaccompanying drawings in which:

FIG. 1 is a schematic perspective view from above of a noise attenuationpanel according to a first embodiment of the invention;

FIG. 2 is a schematic cross section of an end region of the panel shownin FIG. 1, secured to a supporting channel member;

FIGS. 3, 4 and 5 are schematic scrap view cross sections of noiseattenuation panels according to three further embodiments of theinvention; and

FIG. 6 is a schematic cross-section of an aero engine embodying noiseattenuation panels according to the invention.

Referring first to FIGS. 1 and 2, the noise attenuation panel 10comprises a backing sheet 11, a cellular core 12 and inner and outerfacing sheets 13 and 14.

The cellular core 12 comprises a multiplicity of open-ended juxtaposedcells 15 of hexagonal cross section to provide a honeycombconfiguration.

The backing sheet 11 is unperforated and made from an impermeable sheetmaterial, as shown in FIG. 2, and is secured by an epoxy resin adhesiveE1 to the lower face of the cellular core 12.

The inner facing sheet 13 is made from an open square weave fabricformed from a carbon fibre/resin matrix composite material and the weaveis such as to provide apertures constituted by the openings betweenadjacent warp and weft threads of the fabric. The fabric is preferablyso woven as to produce a proportion of open aperture area relative tothe total surface area of the sheet of around 30%. At the same time, thefabric is so woven that a relatively large number of its apertures arecontained within the bounds of each cell 15 of the cellular core 12.

The outer facing sheet 14 comprises a sheet of a porous permeablethermoplastics material produced by powder sintering the thermoplastic.Examples of suitable thermoplastics materials include polyether etherketone, polyaromatic ketone, polyphenylene sulphide, polyamide-imide,thermoplastic polyimide, polyether-imide, polyurethane and polyethylene.

The cuter facing sheet 14 is bonded to the inner facing sheet 13 and theinner facing sheet 13 is, as shown in FIG. 2, secured to the upper faceof the cellular core 12 by means of an epoxy resin adhesive E2.

As best seen in FIG. 2, the cells 15 are provided with drainage slots 16to allow for condensates to drain from the panel 10.

During manufacture of the panel 10, it may be found advantageous to formthe inner woven facing sheet 13 in a partially cured condition and tobring the outer facing sheet 14 into contact with it during a finalcuring step so that the resin serves to bond the two sheets together.The combined sheets 13 and 14 may then be secured to the upper face ofthe cellular core 12 using the epoxy resin adhesive E2.

The epoxy adhesives E1 and E2 may for example be obtained fromCiba-Geigy Plastics & Additives Company Limited of Cambridge, England.Adhesives and resins need not however be epoxy resin adhesives, butcould for example be a phenolic, polyimide or thermoplastics resin.

The backing sheet 11 requires to be imperforate and made of a non-porousimpermeable material and may be made of any of the following materials:

(i) A carbon/thermoplastic composite where for example the thermoplasticis polyether ether ketone, the material being automatically tape woundor hand laid.

(ii) A carbon/epoxy resin.

(iii) An aluminum alloy.

The cellular core 12 is preferably made from a non-porous impermeablesheet of any of the following materials:

(i) A thermoplastic such as polyether ether ketone.

(ii) A polyester fabric/phenolic resin.

(iii) A fiberglass/phenolic resin.

(iv) A NOMEX/phenolic resin (NOMEX being a registered trade mark for anaramid fibre paper impregnated with various resins to produce astructural material. By "aramid is meant an aromatic polyamide polymer).

(v) An aluminium alloy.

The panel 10 is of arcuate form, possibly of double curvature, and isembodied as a structural part of a duct of a nose cowl of a turbofanaero engine, the panel 10 being one of several arcuate panels disposedjust upstream of the fan of the engine. It is, of course, of vitalimportance that the panel does not deteriorate in use and, inparticular, that no part of it becomes detached from its supportingstructure. The structure will usually include supporting channel membersof which only one member 17 is shown in FIG. 2. The panel 10 is securedto the member 17 by bonding the inner facing sheet 13 to an outer faceof a flange 18 of the channel member 17 using a carbon to carbon bond 19and by bonding the backing sheet 11 to the outer face of a flange 20 ofthe channel member 17 using a carbon to carbon bond 21. The gap betweenthe panel 10 and the base 22 of the channel member 17 may be sealed orclosed by use of a mastic 23.

A panel having an outer facing sheet made of a porous thermoplasticsmaterial as described with reference to FIGS. 1 and 2 has been found togive rise to several advantages over the panels of the prior proposals,including the following:

(1) there is a lower flow resistance to high speed air flow, andtherefore the overall power plant efficiency is improved over thatobtained using the previously proposed panels;

(2) the sound attenuation is greater and covers a wider frequency rangethan that of the previously proposed panels;

(3) the thermoplastic component parts do not have the problem of metalgalvanic corrosion;

(4) the panel is lighter than the previously proposed panels;

(5) there is an improved "blade-off" energy absorption compared with thepreviously proposed structures; and

(6) there is an improved appearance.

Referring now to FIG. 3, a panel according to a second embodiment of theinvention is illustrated, in which the cellular core 12 of theembodiment of the invention described with reference to FIGS. 1 and 2 isreplaced by a split cellular core comprising an upper cellular element121 having a multiplicity of open-ended juxtaposed cells 151, a lowercellular element 122 having a further multiplicity of open-endedjuxtaposed cells 152 and a septum sheet 24 which extends across the endsof the cells 152 of the lower cellular element 122 at the upper facethereof and the ends of the cells 151 of the upper cellular element 121at the lower face thereof. The cells of the two cellular elements 121and 122 form identical arrays and are arranged in alignment with eachother.

The backing sheet 11, the inner and outer facing sheets 13 and 14 andthe upper and lower cellular elements 121 and 122 of the panel shown inFIG. 3 are constructed and bonded together in the same manner as thesheet 11, core 12 and facing sheets 13 and 14 of the embodimentdescribed with reference to FIGS. 1 and 2, with the backing sheet 11being secured to the lower face of the lower cellular element 122 andthe inner and outer facing sheets 13 and 14 being-secured to the upperface of the upper cellular element 121.

The septum sheet 24 is likewise bonded to adjacent faces of the cellularelements 121 and 122. It is constructed from a porous permeablethermoplastics material and may take the same form as that of the outerfacing sheet 14 of the panel described with reference to FIGS. 1 and 2.

In a third embodiment of the invention illustrated in FIG. 4, the innerfacing sheet 13 of the panel described with reference to FIGS. 1 and 2is omitted and the outer facing sheet 14, which is made of the porousthermoplastics material, is adhered directly to the upper face of thecellular core 12 using adhesive E2.

In FIG. 5, a fourth embodiment of the invention is illustrated whichcorresponds to the embodiment of the invention described with referenceto FIG. 3, except insofar as the inner facing sheet 13 is omitted andthe cuter facing sheet 14 is adhered direct to the face of the uppercellular element 121 in the manner described for the panel illustratedin FIG. 4.

In yet another embodiment of the invention (not illustrated) the paneldescribed with reference to FIG. 3 may be modified by making the uppercellular element 121 of a porous thermoplastics material and may be madefrom any of the materials proposed for the outer facing sheet 14 of thepanel described with reference to FIGS. 1 and 2.

The septum sheet 24 in the panels illustrated in FIGS. 3 and 5 isdescribed as being made of a porous thermoplastics material. It mayhowever if desired be made of a stainless steel fabric or any of theabove-mentioned materials which are suitable for the inner facing sheet13, the requirement of course being that the sheet is either porous andpermeable, perforated or apertured.

Referring now to FIG. 6, an aero engine 25 is schematically illustratedand includes a turbofan power unit 26 mounted within a nacelle 27suspended from a pylon 32. The nacelle 27 includes a nose cowl 28 havingan outer wall 29 and an inner wall 30. The inner wall 30 is in partformed by noise attenuation panels 10 as described and illustrated withreference to FIGS. 1 and 2. The panels 10 of FIG. 2 are arranged to formpart of the inner wall of the nose cowl 28 in such disposition that theporous thermoplastics sheet 14 forms the wall surface defining the airintake duct 31 for the power unit 26. The panels 10, in thisdisposition, serve to reduce noise created by the high speed flow of airpassing through the duct 31 and into the power unit 26, as well as toreduce noise generated by the fan blades of the unit 26.

It is to be emphasized that the panels 10 in FIG. 6 are not employed toreduce air noise by a reduction of the air speed by passage of the airthrough the panels, but by contrast acoustic attenuation is achievedwithout affecting the speed of the air which generates the noise, thatis to say, the air does not pass through the noise attenuation panels10.

In the aero engine mounting arrangement illustrated in FIG. 6, the powerunit is carried by the wing mounted pylon 32. It will however beappreciated that noise attenuation panels 10 according to the presentinvention may equally well be employed for reducing noise in other aeroengine installations.

I claim:
 1. A noise attenuation panel comprising a backing componentpart, a facing component part and a cellular component part having amultiplicity of open-ended juxtaposed cells, the backing component partextending across the ends of the cells of the cellular component part atthe rear thereof and the facing component part extending across the endsof the cells of the cellular component part at the front thereof,wherein the facing component part comprises an outer facing sheet of aporous permeable powder sintered thermoplastics material.
 2. A panelaccording to claim 1, wherein the facing component part includes aninner facing sheet which is perforated.
 3. A panel according to claim 2,wherein the inner facing sheet is made of an impermeable material.
 4. Apanel according to claim 1, wherein the backing component part isimperforate and made of an impermeable material.
 5. A panel according toany one of claims 1, 2, 3 or 4, wherein the cellular component partcomprises a front cellular element having a multiplicity of open-endedjuxtaposed cells, a rear cellular element having a further multiplicityof open-ended juxtaposed cells and a septum element which extends acrossthe ends the cells of the rear cellular element at the front thereof andthe ends of the cells of the front cellular element at the rear thereofand wherein the septum element is in the form of a sheet of a porouspermeable thermoplastics material.
 6. A panel according to claim 5,wherein the porous permeable thermoplastics sheet forming the septumelement is produced by powder sintering of a thermoplastics material. 7.A panel according to claim 5, wherein the walls of the cells of the rearcellular element are made of an impermeable material and wherein thecells of the front cellular element are made of a porous permeablethermoplastics material.
 8. A panel according to claim 7, wherein thewalls of the cells of the rear cellular element are made of a non-porousthermoplastics material.
 9. A panel according to claim 1, wherein thewalls of the cells of the cellular component part are made of animpermeable material.
 10. A panel according to any one of claims 3, 4 or9, wherein the impermeable material is a non-porous thermoplasticsmaterial.
 11. A panel according to claim 1, wherein the thermoplasticmaterial is polyether ether ketone.
 12. A panel according to claim 1wherein the thermoplastic material is selected from the group consistingof polyaromatic ketone, polyphenylene sulphide, polyamide-imide,thermoplastic polyimide, polyether-imide, polyurethane and polyethylene.13. A noise attenuation panel comprising a backing component part, afacing component part and a cellular component part having amultiplicity of open-ended juxtaposed cells, the backing component partextending across the ends of the cells of the cellular component part atthe rear thereof and the facing component part extending across the endsof the cells of the cellular component part at the front thereof,wherein the facing component part comprises an outer facing sheet of aporous permeable thermoplastics material produced by powder sinteringsaid thermoplastic material.
 14. A noise attenuation panel comprising abacking component part, a facing component part and a cellular componentpart having a multiplicity of open-ended juxtaposed cells, the backingcomponent part extending across the ends of the cells of the cellularcomponent part at the rear thereof and the facing component partextending across the ends of the cells of the cellular component part atthe front thereof, wherein the facing component part comprises a porouspermeable powder sintered thermoplastics outer facing sheet.
 15. A noiseattenuation panel comprising a backing component part, a facingcomponent part and a cellular component part having a multiplicity ofopen-ended juxtaposed cells, the backing component part extending acrossthe ends of the cells of the cellular component part at the rear thereofand the facing component part extending across the ends of the cells ofthe cellular component part at the front thereof, wherein the facingcomponent part comprises a porous permeable outer facing sheet in theform of a body of powder sintered thermoplastics material, the degree ofpowder sintering being such as to make the outer facing sheet porous andpermeable.